Some data processing devices can place one or more functional modules in a normal state to perform normal specified functions or in a low-power state in order to conserve power. Such devices can include a power control device to control application of a supply voltage to the functional modules. When the power control device reduces the supply voltage applied at a functional module, the module is placed into a low-power state, thereby conserving power. However, inclusion of the power control device can also impact the performance of the functional module in the normal mode of operation. Accordingly, a device or method of determining the impact of a power control device on a functional module would be useful.